1. Field of the Invention
The present invention generally relates to a piston ring, which is used in an internal combustion engine, and more particularly to a piston ring, which can effectively prevent an aluminum adhesion (deposition) phenomenon on the piston ring.
2. Description of the Related Art
A pressure ring and an oil ring as piston rings, are installed to a reciprocating type piston. This pressure ring has a function to prevent a phenomenon that high pressure combustion gas flows from the combustion room into the crank room (blow-by phenomenon), and to prevent excess lubrication oil on a cylinder inner wall from flowing into the combustion room. On the other hand, the oil ring has a function to restrain a phenomenon that excess lubrication oil on the cylinder inner wall invades from the crank room into the combustion room, and is consumed there (oil-up phenomenon). As a standard combination of piston rings, there is a combination of three rings, i.e., two pressure rings which consist of a top ring and a second ring, and one oil ring.
Quality required for the piston rings is getting high as the internal combustion engine becomes lighter and the output power thereof increases. A surface treatment for anti-abrasion such as a hard chromium plating treatment, a fused spray treatment and a nitriding treatment, is applied to a bearing surface of a piston ring for the internal combustion engine, as a technique to improve the fatigue strength or durability thereof.
Among those surface treatments, the nitriding treatment especially exhibits an excellent anti-abrasion property, so that it is used as a surface treatment for a piston ring which is used under severe operating conditions.
However, according to the knowledge and earnest researches of the present inventors, although the piston ring on which the nitriding treatment layer is formed, is excellent in the anti-abrasion property, there is a tendency that the abrasion in the ring groove of the piston increases if it is installed to a piston made from aluminum alloy. Further, due to the abrasion in the ring groove of the piston, as shown in FIGS. 3A to 3C, aluminum deposition or adhesion occurs such that aluminum on a groove lower surface 11 of a piston 10 made from aluminum alloy, is adhered on a lower surface 3a of a piston ring 1 (FIG. 3C).
FIGS. 4A to 4C show charts obtained by a surface rugosity testing device of contact probe type, which indicate the changes in the surface condition of an upper surface 12 and the lower surface 11 of the ring groove of the piston. The abscissa represents a position in the circumferential direction of the ring groove of the piston ring, and the ordinate represents the rugosity and undulation height of the ring groove of the piston ring. As shown in FIGS. 4A to 4C, the surface condition of the upper surface 12 and the lower surface 11 of the ring groove of the piston, is changed from a normal condition (FIG. 4A), to a piston groove rugous condition (FIG. 4B), and further to an aluminum deposition condition (FIG. 4C).
FIGS. 5A to 5C, show the mechanism of the aluminum deposition. As shown in those figures, at first, the lower surface 3a of the piston ring 1, and the groove lower surface 11 of the piston 10 made from aluminum alloy, are contacted with each other through an oxide film 8 (which thickness is not greater than 0.2 micrometer) formed on each surface (FIG. 5A). Next, the oxide film 8 is destroyed because the stress of the oxide film 8 at the contact portion becomes locally high, so that Fe in the lower surface 3a of the piston ring 1 and A1 in the groove lower surface 11 of the piston 10 made from aluminum alloy, are bonded (FIG. 5B). Then, aluminum 20 is adhered on the lower surface 3a of the piston ring 1 (FIG. 5C). FIG. 6 shows an enlarged view of the aluminum deposited and adhered portion. In FIG. 6, a reference numeral 20 represents the deposited aluminum, and 21 represents the bonding portion of A1 and Fe.
In accompaniment with up and down reciprocating motions, when the partial abrasion occurs at a certain portion of the groove of the piston, due to the adhesion phenomenon through the above mentioned progress, the amount of oil consumption increases in the internal combustion engine, because the blow-by gas flows, so that the output power of the engine is deteriorated. This phenomenon occurs on the lower side of the ring groove of the piston in a relatively short time period, and has a great influence onto the fatigue strength or the life of the internal combustion engine. Therefore, there may be proposed a remedy against the abrasion of the piston groove.
For example, as one remedy against the abrasion of the piston groove, in order to prevent the piston and the piston ring from directly contacting with each other, an anodic oxidation coating treatment, a plating treatment, a matrix reinforcing treatment (within the piston) may be applied to the piston side. As the remedy with respect to the piston ring side, a phosphate coating treatment, a plating treatment, or a resin coating treatment for coating resin 9 (e.g. defric (made by Kawamura Research Raboratories) coating treatment) on the surfaces of the piston 10 and the piston ring 1 as shown in FIGS. 7A and 7B, may be applied.
The applicant of the present invention have proposed one remedy against the piston groove abrasion, as follows. Namely, anti-abrasion resin such as epoxy resin, fluorine containing resin and polyamide-imide resin, is mixed with molybdenum disulfide, graphite, carbon, boron nitride etc., as solid lubricants. Carbon fibers, glass fibers etc. may be added and mixed to it, as filler, as the occasion demands. Then, it is coated on the upper and lower surfaces of the piston ring by spraying, immersion, electrostatic coating etc, and is baked to form a coating film (Japanese Patent Laid Open Hei 1-307,568).
However, although the above explained remedies for abrasion of the piston groove have some effect to prevent the aluminum deposition at the initial stage in the usage of the piston, they are not adequately effective for middle and long term usage, so that further improvement is demanded.